1. Field of the Invention
The present invention relates to switching controllers for use in switching DC-to-DC converters, in which the duty cycle of each power channel is controlled by a pulse width modulated control signal. More specifically, the invention is a method and circuit for generating a train of pulses whose width is indicative of the nominal power switch on-time needed to produce a DC output potential V.sub.out in response to a DC input potential V.sub.in. The pulse train is useful in a switching controller for a DC-to-DC converter.
2. Description of the Related Art
For convenience, we will use the expression "switching controller" chip below to denote either a controller (implemented as an integrated circuit) which generates power switch control signals for at least one power switch implemented external to the chip (typically multiple power switches, each implemented external to the chip), or a switching "regulator" (implemented as an integrated circuit) which generates such power switch control signals and which also includes at least one power switch implemented on-board the chip (typically multiple power switches, each implemented on-board the chip ). The power switches are typically MOSFET devices.
One type of conventional switching power supply which employs current mode control to achieve output voltage regulation is a DC-to-DC converter including a current mode switching controller chip, and circuitry external to the controller chip which defines one or more power channels (e.g., multiple paralleled power channels). Each power channel includes an inductor and at least one power switch. The controller chip includes a control signal channel for each power channel. Each control signal channel generates a pulse width modulated power switch control signal in response to a feedback signal (a ramped voltage) indicative of the current through the channel's inductor, and a second feedback signal indicative of the DC-to-DC converter's output potential. This allows control of the time-averaged duty cycle of each of the channel's power switches. Typically, a ramped voltage (internally generated in the controller chip) adjusts the effective ramp rate slightly for improved stability through a technique called "slope compensation." The adjusted feedback signal (e.g., one whose value is the difference between the second feedback signal and the internally generated ramped voltage) controls the peak value to which the inductor is allowed to ramp up. Also typically, each pulse width modulated power switch control signal is a binary signal having periodically occurring leading edges, and trailing edges which occur at times determined by the instantaneous value of the feedback signal.
Some DC-to-DC converters include a current mode switching controller chip and buck converter circuitry external to the controller chip. The buck converter circuitry comprises an NMOS transistor (which functions as a power switch, and has a drain coupled to the DC-to-DC converter's input node), an inductor and a current sense resistor (connected in series with the channel of the NMOS transistor), a Schottky diode (coupled between ground and the source of the NMOS transistor), an output capacitor coupled between ground and the output node, and a feedback resistor divider coupled between ground and the output node.
In order to generate a pulse width modulated power switch control signal in response to feedback (or in response to a DC signal which corresponds to a desired DC-to-DC converter output potential V.sub.out), it would be desirable for DC-to-DC converter switching controller chips (of many different types) to include circuitry for generating a train of pulses whose width is indicative of the nominal power switch on-time needed to produce a DC output potential V.sub.out in response to a DC input potential V.sub.in. It would be especially useful for the circuitry to do so in an accurate manner, independently of process variations (i.e., variations in the manufactured characteristics of integrated circuits produced using the same design) and variations in operating temperature.